The invention relates to pneumatic percussive tools, and in particular, to double-piston rock drills which may be used for drilling blast holes in the mining, in construction and other industries.
It is known that one of the major disadvantages of portable rock drills resides in a considerable vibration which is mainly due to the action of alternating air pressure forces on the rock drill housing and to the axial force component in the screw pair of a rotary drive.
Attemps were made to lower vibration of rock drills. The most effective solutions involved the development of double-piston rock drills having a barrel accommodating two pistons moving in opposition to each other during the operation of the tools. At least one of the pistons strikes against the drill steel.
In addition, in order to provide for rotation of the drill steel, one of the pistons has a helical groove which is received in a nut of the rotary drive mounted with an overrunning clutch in the rock drill housing. During the movement, this piston causes rotation of the drill steel. This construction permits the reduction of fluctuation of the resultant of compressed air pressure forces acting on the rock drill housing, but it cannot provide for reduction of the axial force component in the screw pair of the rotary drive acting on the rock drill housing. With fluctuating axial force component in the screw pair acting on the housing, the housing vibrations remains at a considerable level. This impairs the effect of utilization of double-piston rock drills.
Therefore, the problem might be solved by providing a double-piston rock drill in which the action of the axial force of the screw pair of the housing would be eliminated. One of the solutions to the problem might reside in complete elimination of the screw pair with the employment of an independent rotary drive. Thus, FRG patents Nos. 1057037 and 1175184 disclose double-piston rock drills wherein the pistons move in opposition to each other, and the drill steel is rotated by an independent auxillary motor. However, this solution is inexpedient for a portable rock drill, because the use of an additional independent rotary drive results in a heavier tool. This is inadmissible in portable rock drills. It is, therefore, desirable to find another solution to this problem, while retaining a screw rotary drive in the rock drill construction which is compact and provides for lowering the energy of piston strokes upon an increase in the resistance torque at the drill steel. This prevents the drill steel from being jammed during the drilling.